Austenitic alloy steel and article made therefrom



Patented Jan. 21, 1941 UNITED STATES PATENT orrlce Russell :Franks,Niagara Falll,'N. Y., assignor to. Electro Metallur icai Company, acorporation of West Virginia fio Drawing. Application December 14, 1938,

Serial No. 245,593

scum.- (01.75-128) The invention relates to austenitic chromium alloysteels and is particularly concerned with means for improving the hotworking characteristics of such steels.

Chromium steels containing about 12% to 25% chromium, 6% to 20% ofaustenite-promoting metal oi! the group consisting oi nickel, manganese,and mixtures theerof, and carbon in an amount not exceeding about 0.12%,are corrosion resistant, tough, and ductile. To improve certain of thecharacteristics of these steels, for example, their resistance tointergranular corroslon, it has been proposed to add a substantialamount of one or more of the ferrite-promoting, carbide-forming elementsof the group consistmg or titanium, zirconium, vanadium, columbiurn,tantalum, molybdenum and tungsten.

hile such ,elements effectively inhibit intergranular corrosion, theirpresence appears to have a deleterious effect onthe hot workingcharacteristics of the steels. It is believed that the difficulties inhot working, usually manifest by tears, checks, or cracks in the steels,are caused by the partial decomposition of the austenite oi. the steelunder the conditions of high temperature and mechanical work, forming asmall amount of an undesirable ferritic constituent throughout theaustenite matrix. This small proportion of ierritic constituent appearsto detrimentally afiect the hot workability of steels oi the class inquestion.

Measures have been proposed to improve the hot working characteristicsof such steels, but none has been entirely satisfactory. One proposalhas been to increase the carbon content and thus, by promoting a morestable austenite, to restrict both the tendency of the carbideformingelements to form the undesirable ferritic constituent, and the tendencyof the austenite to decompose and form such a constituent during hotworking. This expedient, while improving hot workability, has adeleterious effect, not only on the general corrosion resistance of themetal, but also on its resistance to inter-granular attack. It has alsobeen proposed to promote a more stable austenite by increasing thenickel or manganese content, or combined nickel and manganese content ofthe steel above 20%, but this expedient is only partially effective andis moreover, relatively expensive. The use of a very high proportion ofnickel has the added disadvantage of increasing the hot-stiffness of themetal, thereby increasing the difilculty of working it.

i have found that the addition of relatively small amounts of nitrogento steels containing about 12% to 25% chromium, 6% to 20% of anaustenite-promoting element of the group consisting of nickel,manganese, and mixtures thereof, carbon in an amount less than 0.12%,and a 0 total amount, not over of one or more of the ferrite-promoting,carbide-forming elements 0! the group consisting of titanium, zirconium,vanadium, columbium, tantalum, tungsten, molybdenum, and mixturesthereof considerably improves the hot working and other properties ofsuch steels without impairing their excellent corrosion-resistance orwithout substantially increasing their resistance to deformation at hotworking temperatures. The nitrogen content is 15 uniformly distributedthroughout the steel and should be between 0.05% and 05%, preferablybetween 0.05% and 0.2%. As is customary in steels of this class, coppermay be added in amounts up to about 2.5%. The maximum permy centage ofnitrogen that can be held in stable combination in these alloys dependson their chromium, nickel, and manganese contents. If the chromiumcontent is or over, andthe nickel, manganese, or nickel-plus manganesecontent is low, the nitrogen content may rise to 0.5%, but when thechromium content/is below 20%, and the nickel, manganese, or nickel-plusmanganese content is high, it is advisable to keep the nitrogen contentbelow 0.2%.

l have observed that the nitrogen imparts stability to the austeniticconstituent of the steel, strengthens and otherwise improves theproperties of the ierritic constituents particularly at elevatedtemperatures, and promotes a finegrained structure throughout the steel.It is probable that these effects are responsible, at least in largepart, for the improved hot workability of the steels and for the factthat during the heat treating of such steels the cooling rate necessaryfor a given section, to retain a high proportion of austenite, can beslower than that necessary to retain the same proportion of austenite in.1 steel containing little or no nitrogen but otherwise of similaranalysis. The reduced cooling as rate is particularly valuable inimparting good ductility and toughness to those articles, either cast orwrought, which, owing to their shape, thickness or other factors, couldnot be very rapidly cooled by known practical methods.

Another valuable characteristic resulting from the addition of nitrogento the steels herein described is that the yield point and maximumstrength of the hot-worked steels are increased substantially withoutthe proportionate decrease til in impact strength and ductility whichusually accompanies such improved properties.

This and other improvements in the physical properties of the hot-workedsteels are indicated nitrogen, which counteracts the said adverse effectof the columbium: remainder iron and incidental impurities including notover 0.12% carbon.

in the following table: 2. Hot worked austenitic alloy steel containingComposition (remainder Fe) Tensile test results Percent Percent PercentPercent Percent Percent Percent Percent M 0 ob Mo N Y. P. M. 8. EL RAIzod B. H.

1 17. 5 9 0. 07 None None 0. 04 84 87 59 64 107 128 2.. l8. 5 0 0.09 l.1 None 0. 04 86 90 64 70 101 3 18.0 9 0. 08 l. 1 None 0. ll 48 1(1) 5066 107 4. l8. 3 l0 0. 06 None 2. d 0. 04 37 90 57 73 98 146 6.- 18. 3 100. 08 None 2. 6 0. 10 42 92 58 74 102 156 6.- l8. 2 9. 6 0.06 None 3. 20. 04 40 92 54 73 97 163 7.-- 18. 2 9. l 0. 08 None 3. 2 0. 13 49 99 6674 113 170 N own-Steels No. l to 3 hot worked and water quenched froml,lfi0 C. before testing.

and air cooled from l,i00 C. before testing.

In the above table, the following symbols are used: Y.,P. for yieldpoint in thousands of pounds per square inch; M. S. for maximum stressin thousands of pounds per square inch; per cent EL for percentageelongation in two inches in a standard 0.505 inch tensile sample; percent RA for percentage reduction in area of cross section accompanyingthe elongation; Izod to designate the Izod impact resistance in footpounds; and B. H. for the hardness on the Brineil scale.

The steels of the invention may be satisfactorily hot worked followingthe procedure ordinarily employed in working plain austenitic chromiumnickel steels. The resulting articles are free from the surfaceimperfections usually produced in hot worked austenitic chromium nickelsteels containing ferrite-promoting, carbide-forming elements. Further,the improved hot working and other characteristics of the steels hereindescribed are obtained without deleteriously afl'ecting the strength,toughness, ductility, or corrosion-resistance of the metal.

I claim:

1. Hot worked austenitic alloy steel containing between 12% and 25%chromium; between 6% and 20% nickel, a substantial proportion of theferrite-promoting ingredient columbium, having a favorable eflect on thecorrosion resistance of the steel but an adverse effect on its hotworking properties, but not exceeding 5%; 0.05% to 0.2%

Steels No. 4 to 7 hot worked between 12% and 25% chromium; between 6%and 20% nickel; substantial proportions of the ferrite promotingingredients columbium and molybdenum, having a favorable effect on thecorrosion resistance of the steel but an adverse effect on its hotworking properties, but not exceeding 5% in total sum; 0.05% to 0.2%nitrogen which counteracts the said adverse effect of the columbium andmolybdenum; remainder iron and incidental impurities including not over0.12% carbon.

3. Hot worked corrosion resistant austenitic alloy steel articlecontaining between 12% and 25% chromium; between 6% and 20% of at leastone material, promoting an austenitic structure, selected from the groupconsisting of nickel and manganese; a substantial proportion of at leastone ferrite-promoting ingredient having a favorable efl'ect on thecorrosion resistance of the steel, but an adverse eflect on its hotworking properties, such ingredient or ingredients being in theaggregate not over 5% of the steel and being selected from the groupconsisting of titanium, zirconium, vanadium, columbium, tantalum,molybdenum, and tungsten; between 0.05% and 0.5% nitrogen whichcounteracts the said adverse effect of the ferrite-promotingingredients; the remainder iron and incidental impurities including notover about 0.12% carbon.

RUSSELL FRANKS.

